Re: Litmus test for question from Al Viro

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Oct 02, 2020 at 10:35:45PM -0400, Jon Masters wrote:
> On 10/1/20 12:15 PM, Alan Stern wrote:
> > On Wed, Sep 30, 2020 at 09:51:16PM -0700, Paul E. McKenney wrote:
> > > Hello!
> > > 
> > > Al Viro posted the following query:
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > <viro> fun question regarding barriers, if you have time for that
> > > <viro>         V->A = V->B = 1;
> > > <viro>
> > > <viro> CPU1:
> > > <viro>         to_free = NULL
> > > <viro>         spin_lock(&LOCK)
> > > <viro>         if (!smp_load_acquire(&V->B))
> > > <viro>                 to_free = V
> > > <viro>         V->A = 0
> > > <viro>         spin_unlock(&LOCK)
> > > <viro>         kfree(to_free)
> > > <viro>
> > > <viro> CPU2:
> > > <viro>         to_free = V;
> > > <viro>         if (READ_ONCE(V->A)) {
> > > <viro>                 spin_lock(&LOCK)
> > > <viro>                 if (V->A)
> > > <viro>                         to_free = NULL
> > > <viro>                 smp_store_release(&V->B, 0);
> > > <viro>                 spin_unlock(&LOCK)
> > > <viro>         }
> > > <viro>         kfree(to_free);
> > > <viro> 1) is it guaranteed that V will be freed exactly once and that
> > > 	  no accesses to *V will happen after freeing it?
> > > <viro> 2) do we need smp_store_release() there?  I.e. will anything
> > > 	  break if it's replaced with plain V->B = 0?
> > 
> > Here are my answers to Al's questions:
> > 
> > 1) It is guaranteed that V will be freed exactly once.  It is not
> > guaranteed that no accesses to *V will occur after it is freed, because
> > the test contains a data race.  CPU1's plain "V->A = 0" write races with
> > CPU2's READ_ONCE; if the plain write were replaced with
> > "WRITE_ONCE(V->A, 0)" then the guarantee would hold.  Equally well,
> > CPU1's smp_load_acquire could be replaced with a plain read while the
> > plain write is replaced with smp_store_release.
> > 
> > 2) The smp_store_release in CPU2 is not needed.  Replacing it with a
> > plain V->B = 0 will not break anything.
> 
> This was my interpretation also. I made the mistake of reading this right
> before trying to go to bed the other night and ended up tweeting at Paul
> that I'd regret it if he gave me scary dreams. Thought about it and read
> your write up and it is still exactly how I see it.

Should I have added a "read at your own risk" disclaimer?  ;-)

							Thanx, Paul